Ocean Production
and
CO2 uptake
Fig. 6.6
Recall: Current ocean is gaining Carbon..
OCEAN
Reservoir size: 38000
Flux in: 90
Flux out: 88+0.2=88.2
90 - 88.2 = 1.8 Pg/yr
OCEAN is gaining 1.8 Pg/yr
Sum of the Sources & Sinks of
Anthropogenic carbon
BIOTA & SOILS
Flux in: 120
Flux out: 4+60+55 = 119
Net SINK: 1 Pg/yr
Human SOURCES:
Measured Atmospheric increase:
CALCULTED Carbon SINKS:
Measured carbon SINKS:
OCEAN
Flux in: 90
Flux out: 88+0.2 = 88.2
Net SINK: 1.8 Pg/yr
7.4 ± 1.0 Pg/yr
4.0 Pg/yr
3.4 ± 1.0 Pg/yr
2.8 Pg/yr
Calculated and Measured SINKS
are equal within error
A very large amount of carbon cycling through the
atmosphere goes into the ocean.. But how does it do it?
Outline:
1. Chemistry. How does ocean
absorb CO2 and what happens?
2. The two “pumps” for putting C
in the deep ocean
a) Abiotic (physical) pump
b) Biological pump
What does it take for
plankton to grow?
1. Chemistry
Focus on Atm and Ocean CO2 boxes
CO2 dissolved in ocean water
Is > 50 x all CO2 in atmosphere!
Why?
Because chemistry of
ocean water can hold so
much..
ATM CO2
Ocean
Dissolved CO2
Not to scale..
Questions:
1) why can ocean hold so much CO2?
2) what does putting more CO2 into
ocean do to water chemistry? `
Question: what is PH of water?
• Remember
this stuff?
What is the Ph of “natural” water?
• Natural water (rain included) is slightly acidic
(Ph 5-6)
• CO2 + H2O ===> H2CO3 (carbonic acid)
• Aside: “Acid rain” is much stronger acid, due to SO2
emissions from coal burning- but reaction is exactly the
same! SO2 + H2O ===> H2SO4
(sulfuric acid)
But then there is more chemistry:
• 1) CO2 dissolves in sea water forming
carbonic acid:
CO2 + H2O => H2CO3
• 2) Carbonic acids yields hydrogen ion and
bicarbonate:
H2CO3 => H+ + HCO3-
• 3) Bicarbonate dissociates to another hydrogen ion and
carbonate:
HCO3- =>H+ + CO32-
Together this is called the
“Carbonate Buffer system”
•H2O + CO2 == H2CO3 == H + + HCO3- = 2 H + + CO3-2
• Note each carbon in CO2 that goes into the ocean as one
thing, can turn into three separate forms
• Each form has its own chemical solubility
Carbonate buffer system is central to maintaining the Ocean’s
PH- AND regulating CO2 storage!
Overall:
The Ocean is major Sink of CO2
…Because increasing levels of Atm CO2 have
increased the air-sea gradients.
Atm
Atm
Ocean
Ocean
Passive (chemical) absorption
mechanism
Atm
Ocean
CO2
H2O + CO2 == H2CO3 == H + + HCO3- = 2 H + + CO32
2. The Ocean’s two
“pumps” for CO2
Question:
Why do you need a “pump”
where is the ocean pumping it to…
Recall those “Ocean Layers”
SURFACE OCEAN
THERMOCLINE
DEEP OCEAN
• Surface “mixed layer” ~ 100 meters deep (out of 4500m!!)
IS ONLY PART WHERE CO2 FROM ATM CAN PENETRATE by
mixing..
• ~ Same depth (~100m) is where all the ocean production can
happen!
So:
Why do you need a “pump”
2a. The physical pump
(“passive” CO2 uptake, no
biology involved)
Oceanic Sources and Sinks of Atmospheric
Carbon Dioxide
Oceanic concentration of CO2 depends on temperature, salinity and biological
productivity. CO2 flux into/out of ocean depends on air-sea CO2 difference.
CO2 from A to O
CO2 from O to A
How do you get surface water
down to the deep?
Recall:
total turnover time of ocean = 1000 years
Surface Ocean
residence time = 100 years
Upwelling
Deep
Water
Formation
Deep Cold Ocean
residence time ~ 1000 years
Recall: Residence time is the average amount of time a substance
(in this case water) spends in a reservoir
Recall: Global Ocean “Conveyor Belt”
circulation
So, if ocean is taking up CO2
at surface,
~ how long would it take to
“fill up”? (reach its capacity)
Ultimately- way more C in fossil fuels vs.
what ocean can absorb..
The “passive
(physical) CO2
pump is
effective- but its
slow.
2b. The biological pump
(“active” pump, due to
biological production and
remineralization)
Global Ocean Productivity
Recall:
Life (biosphere) transforms and recycles
carbon from inorganic to organic forms
Autotrophs
Inorganic matter
(oxidized carbon)
CO2
Heterotrophs
Organic matter
(reduced carbon)
CHO
When ocean plankton grow- the surface ocean is
taking up carbon from the atmosphere!
Autotrophs (Primary Producers)
Land
Ocean
Oceanic Primary Producers (Autotrophs)
Oceanic Primary Producers (Autotrophs)
Certain microbes can bloom under right conditions
But still present and productive when not blooming
1. PLANKTON PHOTOSYNTHESIS:
light
CO2 + H2O + PO4 + NO3
(Biolimiting salts = nutrients)
CH2O,P,N + O2
organic tissue + oxygen
NOTE 1: Biolimiting SALTS are used UP,
Oxygen is PRODUCED
NOTE 2: can only happen in the SURFACE !
Result Nutrient “Profiles”
• Major nutrients are USED UP in surface by photosynthesis (eg:
Phosphate (PO4), Nitrate (NO3), Silicate)
• BUT replaced in deep ocean by remineralization of sinking particles!
The Plankton’s Catch-22
Where are the nutrients? Where is the Light?
Recall: “Stratification”
Ocean is layered by density
“MIXED LAYER” is upper layer of water totally
mixed up by the wind.. the deeper the mixed layer is,
more of those deep- nutrients can reach the surface!
Result:
MOST OF SURFACE OCEAN
PRODUCTION IS
“NUTRIENT LIMITED”
This means, that the main thing which controls overall
plankton production* is AVAILABLE NUTRIENTS in
surface water
(*and therefore the amount of CO2 that can be turned into
plankton biomass)
Recall “upwelling” ?
Effect of Coastal UPWELLING
This explains why
upwelling areas are
so intensely
productive for
fisheries- but also
entire food chains..
Higher latitudes: Highly productive during spring
and summer- (when enough light)
Mid-Lower latitudes: More Constant,
Central areas are “deserts” (no upwelling)
Photosynthesis Summary
-Carried out by Autotrophs
(aka Primary Producers)
-Inorganic Carbon  Organic Carbon
-At the Surface (LIGHT!)
-Limited by nutrient availability
(PO4, NO3)
What becomes of the Carbon next?
Oceanic Consumers
Oceanic Consumers
2. PARTICLE SINKING to depth
Feeding by
Heterotrophs
Wastes and
remains..
3. Respiration at depth
( Animals & bacteria)
CH2O,P,N + O2
organic tissue + oxygen
CO2 + H2O + PO4 + NO3
(Biolimiting salts = nutrients)
NOTE 1: Oxygen is USED UP, Biolimiting SALTS (and CO2) are PRODUCED
This PROCESS is called:
REMINERALIZATION - because “mineral” nutrients
Salts are replaced into the deeper sea
CO2
light
PLANKTON
PHOTOSYNTHESIS
FEEDING BY
HETEROTROPHS
Surface
Deep
PARTICLE
SINKING
CO2 + H2O + PO4 + NO3
RESPIRATION
How do we explain this profile?
-Respiration uses O2 – Creates Oxygen Minimum
-Upwelling leads to higher surface productivity,
which leads to more respiration beneath it
The “Biological Pump”
•C removed on time scale of
plankton bloom (=weeks)
• How long does the C stay down
there?
•Depends on how deep it can get..
The “Martin curve”
•Attenuation of
sinking Particles
(plankton remains ) is
approximately
exponential with
depth
•Most organic tissue
is converted back into
CO2 by 500m
•Almost all (>90%) by
1000m
POC Attenuation
So what controls the effectiveness of
“Biological Pump” ?
•1) Amount of Plankton production
•Nutrients available (NO3, PO4, Fe, Si)
2) Ecosystem structure (= Plankton
types)
* All plankton production is NOT
equal.. (more on this later)
Biological Ocean C-fixation: Summary
•Plankton “draw down” atm.
CO2, fix it into biomass
•This is called “plankton
production”
•Plankton production is
limited mostly by available
nutrients
 Available surface nutrients are main limit on
amount of CO2 that can be removed from Atm.
Biological uptake
Much, Much faster than 1000yrs!
CO2
Biological “Pump”
SURFACE OCEAN
Plankton
Sinking Organic matter
reduced carbon (CHO)
DEEP OCEAN
Heterotrophic
bacteria
CO2
Ocean
“Biological
Pump”
•1) Surface
Plankton fix
carbon
•2) Wastes SINK
•3) Deep bacteria
remineralize OM
Overall, the biological pump:
•Is much faster than abiotic pump- can move Carbon
into mid-ocean in matter of WEEKS.
•Is limited by nutrient availability in open sea
•Ultimately- still cannot mostly reach the “true” deep
ocean
Overall, the biological pump:
 If enough nutrients*.. Then might be a
“short term” fix- ie, sequester carbon on
hundred year time-scales (not full 1000
yrs..)
•Long term: still limited by ocean’s ability
to absorb CO2
* Basis for lots of Geo-engineering schemes.. How to put more
nutrients into the ocean?
Ultimately- cannot escape the relative
reservoir sizes..
The “passive
(physical) CO2
pump
OR
the biological
pump can only be
effective fairly
short (geologically
speaking..)time
frames..